Resource identifiers, such as uniform resource identifiers, are often catalogued (e.g., bookmarked). Tags, such as keywords, are sometimes associated with a resource identifier and stored along with the resource identifier. Each tag, however, is usually considered to be unrelated to any other tags. Thus, if resource identifiers are categorized based on tags associated with the resource identifiers, then each tag will only correlate to a single category. As a result, any categorized view of resource identifiers will be flat (e.g., without any relationships between the categories).